GNU CLISP is an ANSI Common Lisp implementation
with an interpreter, compiler, debugger, object
system (CLOS, MOP), sockets, fast bignums, arbitrary precision floats, and a foreign language interface that runs on most Unix variants and Win32.

CMUCL is a free, high performance implementation of the Common Lisp programming language which runs on most major Unix platforms. It mainly conforms to the ANSI Common Lisp standard. CMUCL provides a sophisticated native code compiler; a powerful foreign function interface; an implementation of CLOS; the Common Lisp Object System; which includes multimethods and a metaobject protocol; a source-level debugger and code profiler; and an Emacs-like editor implemented in Common Lisp. CMUCL is maintained by a team of volunteers collaborating over the Internet, and is mostly in the public domain.

A series is a data structure much like a sequence, with similar kinds of operations. The difference is that in many situations, operations on series may be composed functionally and yet execute iteratively, without the need to construct intermediate series values explicitly. In this manner, series provide both the clarity of a functional programming style and the efficiency of an iterative programming style. Series is the culmination of many years of design and use of this approach, during which some 100,000 lines of application code have been written (by about half a dozen people over the course of seven years) using the series facility in nearly all iteration situations. This includes one large system (KBEmacs) of over 40,000 lines of code. In a nutshell: Think "Efficient MAPCAR". SERIES translates functional-style expressions into efficient loops.

Steel Bank Common Lisp is a development
environment for Common Lisp, with excellent
support for the ANSI standard: garbage collection,
lexical closures, powerful macros, strong dynamic
typing, incremental compilation, and the famous
Common Lisp Object System (multimethods and all).
It also includes many extensions, such as native
threads, socket support, a statistical profiler,
programmable streams, and more. These are all
available through an integrated, interactive
native compiler which feels like an interpreter.
SBCL is unique in being a multiplatform native
compiler which bootstraps itself completely from
source, using a C compiler and any other ANSI
Common Lisp implementation.

ILISP is a package that is designed to integrate
various Lisp implementations (mostly Common Lisp
systems and various Scheme dialects, including
Guile) within Emacs (or XEmacs). ILISP runs an
inferior Lisp process (in Emacs parlance) and
provides a specialized set of commands, key
bindings, and menus to ease the interaction with
it. ILISP commands access the underlying Lisp
process and provide ways to make the editing,
compilation, and execution of Lisp programs much
easier.

OpenMCL is an Open Source version of Digitool's excellent Macintosh Common Lisp (MCL) implementation, which runs on LinuxPPC and MacOS X. It features a native code compiler, multithreading support, and good ANSI CL compliance.

Jabberwocky is a Lisp IDE containing a Lisp-aware editor with syntax highlighting, parentheses matching, a source analyzer, indentation, a source level debugger, a project explorer, and an interaction buffer. It is the replacement for the Lisp Debug project.